The instant invention is a process for the carbothermic reduction of silicon dioxide to silicon metal. The process employs a substantially closed, direct current, submerged-arc furnace.
Typically, silicon is produced in an open, alternating current arc furnace by the carbothermic reduction of silicon dioxide. The overall reduction reaction can be represented as: EQU SiO.sub.2 +2C=Si+2CO (1)
The two key intermediates essential for the production of silicon are silicon monoxide and silicon carbide. The reaction being: EQU SiO+SiC=2Si+CO (2)
Silicon monoxide is produced by the reaction of silicon dioxide with carbon, silicon carbide, or silicon, according to the following reactions: EQU SiO.sub.2 +SiC=SiO+CO+Si (3) EQU SiO.sub.2 +C=SiO+CO (4) EQU 2SiO.sub.2 +SiC=3SiO+CO (5) EQU SiO.sub.2 +Si=2SiO (6)
Silicon carbide is produced by the reaction of carbon with silicon dioxide or silicon monoxide, as shown below: EQU SiO.sub.2 +3C=SiC+2CO (7) EQU SiO+2C=SiC+CO (8)
In a typical submerged-arc furnace process a near stoichiometric mixture of carbon and silica is added to the top of the furnace. As the mixture descends in the furnace, silicon monoxide and silicon carbide are formed according to reactions (3) through (8). Production of excessive silicon monoxide that does not react with carbon or condense in the furnace results in loss of silicon. The most dominant reaction for production of silicon monoxide is by the equimolar reaction of silica with silicon carbide according to reaction (3). The silicon monoxide, formed in the cave zone of the furnace, ascends through the charge where, ideally, it is trapped by reacting with carbon to form silicon carbide according to reaction (8).
Present alternating current (AC) arc furnaces require high energy input for silicon production and are very inefficient from the standpoint of energy utilization. Only about 31 percent of the total energy input to the furnace, both in the form of electrical energy and chemical energy from the reductants, is used for reduction of silica. The remaining 69% of the energy is lost.
The present invention provides a potentially more energy efficient process for producing silicon metal. The use of a direct current (DC) power source, which is more controllable, results in a more efficient furnace, than with AC power. The substantially closed configuration of the furnace minimizes loss of input energy as silicon monoxide and carbon monoxide gases.
Enger et al., U.S. Pat. No. 3,887,359, issued June 3, 1975, describes a process for the carbothermic reduction of silicon dioxide. The process involves feeding the silicon dioxide into the furnace separate from the carbon reducing agent such as to create separate zones in the furnace for each. By-product gases from the reduction of silicon dioxide are passed through one or more zones rich in the carbon reducing agent. Enger et al. suggest the possibility of using alternating current as well as direct current for supply of energy to the process. The furnaces described were open-top furnaces.
Goins et al., U.S. Pat. No. 4,865,643, issued Sept. 12, 1989, describes a smelting process for making elemental silicon in a direct current arc furnace. The furnace configurations described by Goins et al. are open-top furnaces that employ either a hollow electrode or vent tubes within the arc-zone to remove by-product carbon monoxide and silicon monoxide gases.
Herold et al., U.S. Pat. No. 4,450,130, issued May 22, 1984, describes a process for the recovery of combustible gases in an electrometallurgy furnace. The described furnace comprises an external metal casing and an internal refractory lining in which at least one oxidized compound is reduced by means of carbon. The oxide compound and the carbon source are introduced into the furnace in the form of a divided charge which moves progressively down toward the reaction region, passing through a sintering region. The combustible gases produced in the reaction region are collected by suction, by means of a plurality of apertures provided in the external metal casing and the internal refractory lining. The apertures are located at a level corresponding to the lower portion of the divided charge before it passes into the sintering region. Herold et al. states the invention was specifically designed for entirely open furnaces, but there is nothing to prevent the invention being carried into effect in closed or semi-closed furnaces. The type of current used to heat the described furnaces is not specified.